One of the nicest (IMHO) cross-platform WYSIWYG XML editors has gone open source. From the mailing list announcement:"Spread the news! Please mention Serna has gone open source in your blogs [...]"Well, since you asked nicely - OK then. Here are the usual links:

If you use DITA and/or DocBook a lot you really should try Serna, as it has excellent support for both formats. Please note that some functionality (but not a lot) is missing from the free version, see this comparison.